This invention relates to optical elements and more particularly to birefringent structures such as waveplates, diffraction gratings and holograms and to specific methods of fabrication. Anistotropic diffraction is a potentially valuable tool for manipulating optical information. Waveplates and diffraction gratings are among the fundamental tools for optics and spectroscopy.
Optical waveplates are used to control polarization of optical energy. They are conventionally manufactured from birefringent material. Two types of birefringent materials have been used: naturally-occurring crystals and for low-demand applications, stretched polymer films. In a stretched polymer film, the entire film is effected by the manufacturing process so that intricate patterns are impractically difficult to register therein.
With the advent of the laser, a diffraction pattern can be registered in a photo-sensitive medium through illumination by the interference pattern of two coherent plane waves of coherent radiation to cause a transformation in the medium. If the illumination consists of the superpositions of many plane waves, complex interference patterns can be registered in the medium, and the original wave fronts can be reconstructed by subsequent illumination of suitable coherence and pattern. This interference is a fundamental basis of the art of holography.
A grating can be optically-registered in a medium as a variation in transmission amplitude, as a variation in phase or both. Variations in transmission amplitude may be effected by altering absorption in the medium, and variations in phase may be effected by altering the refractive index. Phase-type gratings give the largest diffraction efficiencies.
Various photo-sensitive media have been used for fabrication of holograms and gratings. Such media includes silver halide emulsions, photo-refractive materials, dichromated gelatin, photoresist and formable polymers. These media are capable of forming a permanent record of light intensity by appropriate exposure to illumination and chemical development. When applied in a holographic configuration, the medium registers, in an interference pattern, information about the relative phases of the incident wave fronts. However, the medium is generally incapable of retaining information about the polarization state of the exposing illumination.
Polarization-sensitive gratings and light gates have been fabricated using liquid crystal materials. However, the fabrication of the device employing such materials is an intricate process, and the materials which are used are in a liquid state.
Some photorefractive media are known which are able to record holograms which are sensitive to polarization. Such media include photorefractive crystals. Photorefractive crystals are typically very expensive (on the order of hundreds of dollars each) and require relatively long exposure times (on the orders of milliseconds to seconds) in order to register the phasesensitive patterns.
What is needed is a technique and material suited to register the polarization state of exposing illumination which is relatively inexpensive, which is capable of registration of such information in very short times (on the order of nanoseconds), and which is suitable for birefringent waveplate, grating and holographic applications, as well as for any of a variety of applications of the birefringence.